The present invention relates generally to systems and methods for the application of corrosion resistant coatings to metallic or non-metallic substrates, and more particularly to a multilayer polymer composite coating and method for applying the coating using polymer powders.
Conventional methods for applying protective polymer coatings to metallic substrates include the high temperature, high velocity polymer powder spray. The resultant coatings, however, are often characterized by ineffective adherence to the substrate and/or subsequent coating layers and consequent peeling of the coating, loss of desired properties such as bonding to another material, appearance, or corrosion protection of a metallic substrate. The problem with peeling is especially acute in the application of multiple coatings. For example, in a typical application of a polyolefin coating using high velocity (600 miles per hour (mph)) thermal powder spray onto a steel (1010 cold rolled) substrate, a high velocity thermal sprayed polyfluoride powder resin thereafter applied over the polyolefin coating peeled almost immediately. In other examples, a second polyolefin coating applied over the same previously high velocity thermal sprayed polyolefin resin separated from the first coating during environmental exposure and subsequently peeled off. Similar deficiencies in coatings applied using other polymers have been noted. In accordance with governing principles of the invention, it was hypothesized that the failures resulted from mismatch in thermal expansion coefficients of the two polymer coatings and/or lack of chemical bonding sites or surface bonding and/or slip reducing structure at the interface of the two coatings.
The invention solves or substantially reduces in critical importance problems in the prior art by providing a multi-layer polymer composite coating and method for applying adherent polymer coatings using powders containing two non-polar or slightly polar resins or a non-polar or slightly polar resin and a polar thermoplastic or thermoset resin. The substrate or resin surface of the first coating may be roughened by embedding particles into the substrate or soft first coating. The substrate or coating may be softened by heating to assist embedment and fusion with the particles. The particles can be a single type or mixture of cold to hot, metal, ceramic mineral, or corrosion inhibiting powders, such as stainless steel, aluminum, boron nitride, silica, zinc or zinc plus di-iron phosphate. Additionally or alternatively, filler powder may be added to the first coating plastic powder before or during the spraying process.
The coefficient of thermal expansion of the selected plastic-filler combination of the first and subsequent coating would be selected to improve the match with the coefficient of thermal expansion of the substrate or subsequent filled or unfilled second coating resin. One preferred powder spray process, high velocity thermal spray, drives the first coating deep into the existing and developed structure of the substrate or prepared resin coating, resulting in excellent grip and adhesion to endure the strains of temperature cycling caused by the difference in coefficients of thermal expansion between the substrate and first or subsequent coating. A preferred method of applying the powders to the soft resin is with sand blasting techniques. Velocity and temperature are adjusted to obtain embedment of the particles. A wet or very soft resin requires minimal velocity. For example, a still hot fluidized bed applied nonpolar resin is coated with zinc powder and subsequently overcoated, or, an electrostatically applied slightly polar, partially cured (soft), resin is coated with silica particles and subsequently overcoated. The particles can be applied with dipping in a fluidized bed, air assisted spray, passing through a gravity curtain of particles, etc.
The invention provides low cost, multi-layer polymer coatings having improved corrosion resistance, adjustable colors and high toughness as compared to conventional coatings. The invention allows multi-layer coatings of a low cost primer coat with excellent corrosion properties overlaid with a thin higher cost layer having other desired physical properties (e.g., improved ultraviolet light resistance) and superior moisture, temperature cycling, gloss, hardness and excellent scratch resistance.
It is therefore a principal object of the invention to provide improved corrosion resistant coatings for metallic or non-metallic substrates.
It is another object of the invention to provide corrosion resistant polymer coatings for metallic or non-metallic substrates.
It is another object of the invention to provide a coating that is exceptionally resistant to undercutting corrosion or delamination from the substrate.
It is another object of the invention to provide a multilayer coating that is exceptionally resistant to internal delamination.
It is another object of the invention to provide a coating that is exceptionally resistant to undercutting corrosion or delamination from an embedded matrix.
It is another object of the invention to provide multi-layer corrosion resistant polymer coatings for substrates.
It is another object of the invention to provide low-cost corrosion resistant polymer coatings.
It is another object of the invention to provide active corrosion protection to coatings, while providing exceptional resistance to undercutting corrosion or delamination from the substrate.
It is another object of the invention to provide a multi-layer corrosion resistant coating for a substrate wherein the coefficients of thermal expansion of the layers are substantially matched in order to avoid separation of the layers or peeling of the multi-layer coating from the substrate.
It is a further object of the invention to provide peel resistant polymer coatings having improved toughness, hardness, shear strength, creep resistance, scratch resistance, gloss, appearance and weathering resistance.
It is a further object of the invention to provide a corrosion resistant coating using non-polar and non-polarizable polymers.
It is a further object of the invention to provide an improved method for applying corrosion resistant polymer coatings.
It is another object of the invention to provide corrosion resistant polymer coatings for environmental protection of equipment components, vehicle parts, weapons, concrete reinforcement steel and building structures.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, multi-layer corrosion resistant polymer composite coatings are described wherein a first coating layer is applied by high velocity high temperature spray, thermal spray, plasma spray, fluidized bed, hot melt or electrostatic spray onto an unprepared, particle embedded or alternatively prepared substrate, and the outer surface of the still warm first layer is roughened by embedding particles therein or the outer surface is roughened mechanically, and a second coating layer is applied thereover. Filler powder may be added to one or both of the layers to substantially match the thermal expansion coefficients of the two layers.